1. Field of the Invention
The present invention relates to an imaging lens. In particular, the present invention relates to an imaging lens having a two-lens structure that is capable of size and weight reduction, improved optical performance, and improved productivity. The imaging lens is used in an image-taking device that forms images of objects, such as scenery and human figures, on an image-taking surface of a solid image sensor element, such as a charge-coupled device (CCD), a complementary metal oxide semiconductor (CMOS), etc. The solid image sensor element is mounted on a portable computer, a television phone, a portable phone, and the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for cameras that utilize a solid image sensor element, such as the CCD, the CMOS, or the like, that is mounted on a portable phone, a portable computer, a television phone, and the like. It is demanded that a camera such as this is small and light because the camera is required to be mounted on a limited installation space.
Therefore, it is also necessary for the imaging lens used in such cameras to be similarly small and light. Conventionally, a single-lens structure lens system using a single lens has been used as such an imaging lens.
A single-lens structure lens system such as this is can sufficiently handle a solid image sensor element having a resolution of about 110-thousand pixels, called common intermediate format (CIF). However, the single-lens structure lens system cannot fully demonstrate the resolution capability of a solid image sensor element made available in recent years, referred to as a video graphics array (VGA), having a high resolution of about 300-thousand pixels or a recent solid image sensor element having a higher resolution exceeding one million pixels.
Therefore, in recent years, various two-lens structure lens systems and three-lens structure lens systems having superior optical performance compared to the single-lens structure lens system are being proposed.
In this case, in three-lens structure lens system, aberrations leading to deterioration in optical performance can be effectively corrected, thereby allowing significantly high optical performance. However, the three-lens structure lens system has many components, making size and weight reduction difficult. There is also a problem in that, because each constituent component requires high precision, manufacturing costs also become high.
On the other hand, although optical performance that is as high as that of the three-lens structure lens system cannot be expected from the two-lens structure lens system, higher optical performance than that of the single-lens structure lens system can be obtained. Therefore, the two-lens structure lens system can be considered a suitable lens system for a compact and high-resolution solid image sensor element.
As a two-lens structure lens system such as this, numerous lens systems combining a negative lens and a positive lens, referred to as a retrofocus-type, have been proposed. However, in a retrofocus-type lens system such as this, although costs can be reduced by a reduction in the number of components, the back focus distance increases. Therefore, from the perspective of the structure of the retrofocus-type lens system, the same degree of size and weight reduction as that of the single-lens structure lens system is practically impossible.
As another two-lens structure lens system, there is a lens system combining a negative lens and a positive lens, referred to as a telephoto-type. However, a telephoto-type lens system such as this had originally been developed for silver-halide photography. Therefore, the back focus distance is too short. The lens system also has problems regarding telecentricity. Therefore, it is difficult to use the telephoto-type lens system as is as an imaging lens for the solid image sensor element.
Moreover, conventionally, in the two-lens structure and the three-lens structure lens systems, a mainstream configuration is that in which a diaphragm is disposed between two lenses adjacent to each other in the optical axis direction (refer to, for example, Patent Literatures 1 and 2).    [Patent Literature 1] Japanese Patent Unexamined Publication 2004-163850    [Patent Literature 2] Japanese Patent Unexamined Publication 2004-170460
In recent years, the demand is increasing for further improvement in optical performance, in addition to size and weight reduction. However, in the configuration in which the diaphragm is disposed between two lenses, as in the imaging lenses described in Patent Literatures 1 and 2, achieving both size and weight reduction and further improvement in optical performance is difficult. Furthermore, alignment with the characteristics of the sensor (incident angle to the sensor) is difficult.